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SITE INVESTIGATIONS

Part 1:- DRJLLING TECHNIQUES AND SAMPLING

GENBRALPurpose of Site Investigation

A subsoil investigation is an essential preliminary to theconstruction of all civil engineering works, the purposes of suchinvestigations being:(a) To assess the general suitability of the site for the

proposed works.(b) To enable an adequate and economic design to be

prepared.(c) To foresee and provide against difficulties which may

arise during construction due to ground and other localconditions.Each foundation presents a separate and distinct

problem, and no standard plan for conducting an investigation canbe developed. Thus only abroad outline ofgeneral requirementsand techniques which will be given as a guide on specificinvestigations.

Objectives of a Site Investigation(a) Toexplorefoundationareasinsufficientdetailtopermit

the preparation of maps and profiles, showing thecharacter, thickness, inclination and sequence of allsignificant soil or rock strata and the ground waterconditions through the deposit.

(b) To obtain representative samples ofall significant soil orrock strata for classification and testing purposes.

(c) To test both in the laboratory and in situ as applicablesoil or rock samples fortheirengineering properties suchas shear strength, consolidation, and permeability, inorderto determine the most suitable desigrr and mode ofconstruction of the proposed structure.It is essential to remember that foundation investisation

is a "learn as you go" process.

Geological InformationBasic geological information gives an overall picture of

site conditions but cannot generally be used for detailed planning,e.g. If the geological maps show a limestone as bedrock and theborehole base is in granite, one should be suspicious that a largeboulder is placed between the bottom ofthe borehole and bedrock.Similar use may be made of aerial photos and agricultural soilmaps, etc.

Site ReconnaissanreThe topography, surface water and vegetation should be

examined. Inspect nearby quaries, cuttings, escarpments, etc.Inspect any nearby buildings. Look into the history ofthe site,previous usage may be important. If in a mining area inquire intothe location ofthe underground workings, shafts, etc.

Requirements for Boring Layout and DepthWhile there are no set rules for boring layout and depth

Tables I and 2 give a guide to these requirements respectively.

INVESTIGATION AND BORING METHODS

IntroductionMany different techniques are available for site

investigation. The method employed will depend on many factorssuch as depth required, area to be covered, ease of access, etc. Onlarge jobs preliminary borings are used to fumish overall subsoilsurveys followed by final borings so soil or rock profiles may bedetermined at the most useful orientations. In general, explorationcontracts should be open ended so that intermediate borings maybe added in areas that prove to be critical.

Soil and Rock DrillThis is the most common tlpe of boring method used in

North America where large borehole depths are needed" It can beskid, trailer or truck mounted. It is very versatile, capable ofdrilling with augers, rock bits wash water or drilling fluid and oftenhas a reverse gear that backs augers or unscrews drill strings asneeded. A built in hydraulic pump drive and a hydraulic slide baseare used to push the sampler into cohesive soils. Thus the soil androck drill incorporates many ofthe techniques described later.

Rotary Core DrillingWhere it is convenient to do so, healy foundations are

normaily carried to bedrock and in order to provide samples ofrock for examination and testing, rotary coring methods have to beadopted. Three types of tools are commonly used, industrialdiamond, shot and steel or tungsten toothed cutters. Diamonds aregenerally the fastest, and provide the best core, but may beuneconomic due to the high cost of the crown bit. Standarddiamond core bits give cores between 19 mm and 114 mmdiameter. A large variety of coring barrels are available, varyingfrom a simple single tube banel, to complex triple tube barrels withsplit inner tubes and plastic core sleeves. The more conplexbarrels are used to retain the softer more weaklv cemented. or theheavily fractured rocks.

In order to differentiate rock from boulders, mostspecifications require drilling to be carried 3 m into rock. Thisshould be considered a minimum. Many boulders have beenencountered appreciably larger than this. Care should also be takento compare the cores from different boreholes, and also examinegeological records.

Shell and Auger BoringThis method, which is not very often used in North

Americ4 is quite common in some countries. The shell, a samplerwith a flap (trap) valve or sample (basket) retainer for use in sandsand gravels, and auger, for use in clays and other cohesivematerials is commonly used in the United Kingdom for siteinvestigation. The method is versatile, capable ofdrilling tkoughclays, sands, gravels, and also to some extent in harder materialssuch as soft rocks. Adequate depths can be achieved in the softermaterials for almost any type of foundation investigation. The rateof penetration and sensitivity of the boring is dependent on theweight ofthe equipment used, and this should be varied to meet theparticular requirements. The lighter the equipment the moreaccurate the information, and less disturbed the Ftround.

In clays and other cohesive materials, an auger is

Site Investigations - GEOTECHNICAL ENGINEERING-1997 -- by G.P. Raymond@

generally employed.

In sands and gravels, a shell is used. This tool which isa tube fitted with a cutting shoe and hinged flap valve, is surged upand down in sufficient water to at least cover the tool, the pumpingaction forcing material past the flap valve.

A shell and auger borehole is usually lined by drivingsteel tubes. The hole is normally drilled somewhat in advance ofthe casing. Shell and auger borings for site investigation work, arenormally carried out using 150 mm, 200 mm, 250 mm oroccasionally 300 mm diameter casing. Hard strata, such as softrock can be penetrated by pounding with a chisel, and using a shellto bail the broken fragrnents from the borehole.

The rigs used vary in size, probability and winchcapacity, depending on the type ofstrat4 and the depth ofboringrequired. Typical winch capacities (i.e. the pull which can beexerted on the rope) would be between 3 and 10 kN for siteinvestigation work. Manual operation of the rig is essential to give"feel" to the boring, which is important in recognizing stratachanges, etc.

Wash BoringBorings can be made in most alluvial strata by a wash

boring technique. A shopping bit on a string ofrods is used insidea casirrg, soft strata being washed out below the casing and carriedto the surface by a jet of water passing through the rods and bit,and returning inside the casing. Firmer materials are penetrated bychopping with the bit, and chopped particles being carried to thesurface by the flow ofwater. The casing can usually be agitateddown by turning, as boring proceeds, but it may be necessary todrive it. Samples can be obtained and in situ tests made throughthe casing from time to time. In this method of boring,unlesscontinuous samples are taken,which defeats the main object ofthetechnique, speed, the only evidence ofthe strata being penetratedis the very fine soil particles being carried to the surface by theflow of water. Wash borings are normally made using casingbetween 50 mm and 150 mm diameter, above this size, the pumpunit required is generally too large. The technique is generallyused as a fast and consequently cheap method ofsupplementinginformation obtained liom a series of dry sample borings. It isparticularly useful for obtaining samples or carrying out in situtests at some depth in know strata" e.g. in a clay layer, below a sandstratum. Disturbance of the ground by the water jet may in somecases extend two feet or more below the casing, and care should betaken in sampling and testing to ensure that this is not carried outin the disturbed area. The use of wash boring without adequate drysample boring control should be avoided.

JettingJettings are made in a similar manner to wash borings,

except that no casing is used. The wash water forms and holdsopen to some extent, a hole along the rods, and washings arebrought to the surface through this. As soon as the flow ofwaterstops, collapse ofthe hole occurs in most soils, and consequently,sampling and testing can not be carried out. The washingsbrought to the surface can not be taken as representative of thestrata being penetrated, as collapse ofthe hole continually takesplace. In consequence, the only purpose ofjettings can be to locatea hard stratum which can not be easily penetrated, e.g. rock.Unless very good control by other boring methods is available, and

an area is knorvn to be fiee from boulders or even large gravel,jettings can be very misleading and should never be used as a solemeans of site investigation. The method is cheap and quick, andcan under proper control and in the correct circumstances providedvaluable supple,ment to other methods of investigation.

Mechanical AugerA large variety of size and type are available. Basic

types are:(a) Plate Auger. Used in strata which will stand

unsupported. It is necessary to pull out every foot toexamine cuttings. Depth limited by length of kelly bar(generally 6 m).

(b) Continuous Flight Auger. A spiral continuous flight isused to transler the soil to the surface. Identification ofstrata changes is difficult. Useful in proving knownstrata.

(c) Hollow Flight Auger. A continuous spiral around a tubeis used to transfer cuttings to the surface. A plug andspade auger device can be used to drill soil below thecontrol tube, or a continuous sample can be taken in acentral sampling banel, or undisturbed samples drivenahead through the tube. This is frequently a slowprocess, and due to the very great torque required todrive the auger may be uneconomic. This method islargely experimental at the moment.

(d) Bucket or Grab Auger. This type ofauger drills a largediameter hole, with or without casing. A large plant isinvolved, and it is infrequently used in investigationwork.

Post-Hole-AugerThe simplest tlpe of boring tool, very economical

produces negligible disturbance ofthe soil. Limited in applicationand depth (about 6 m in soft clays). Small hand pushedundisturbed samples may be obtained in soft cohesive soils.

Tests PitsTest pits are in most cases the best method of

investigation,but the cost is generally prohibitive. ln previous soilground water lowering is necessary below the water table.Undisturbed sarnples in cohesive soils are easily obtained.

Borings over WaterBorings over water for maritime or river works present

their own problems. Not least of these is the continual movementofvessels due to time, currents or wind action. ln consequence,wherever it is economical to do so, fixed stations should be used inpreference to boring craft. Ifborings are to be made in deep waterwith strong currents, it is liequently necessary to employ two ormore strings ofcasing inside one another to provide the necessarystifftress and strength. Where diamond core drilling has to be madefrom a floating craft, it is frequently necessary to use a sliding kellyto provide drive and at the same time allow vertical movement ofthe craft. Heavy anchors must be used and given constant attentionto prevent dragging, and to maintain drill central over the borehole.Sampling and in situ testing are also difficult if the boring craft ismoving, and care must be taken to ensure that the results obtainedare accurate.

UNDISTURBED SAMPLINGCleaning Borehole

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Al1 boreholes should be cleaned prior to undisturbedsampling. If cleaned out with jet bits only those bits which deflectthe water or drilling fluid upwards should be used in the bottom150 mm of the borehole. Open end drill rod, sampling spoon, sandpump or bailer should not be used within 300 mm of the intendedtop of sample. If a casing is used the borehole should be cleanedout at least to the tip ofthe casing and preferably 150 mm deeper.Ttre casing should preferably be pushed rather than driven andshould never go below the bottom ofthe borehole ifsampling is tobe done. Where jetting is used for cleaning, special attention isneeded to see that coirse washed material is removed beforesampling. Viscous drilling fluid is prefened to water whensampling saturated cohesionless sands. It may also be necessarywhen sampling other soil t)?es to prevent sample loss, excessswell, and disturbance in the vicinity of the sample. As soon aspossible after the borehole is cleaned, the undisturbed sampleshould be obtained. Ifa delay is unavoidable, recleaning should beperformed.

Undisturbed sampling cohesive soilsIt is notpractical to take completelyundisturbed samples

ofclay, as the removal ofthe sample from the ground involves achange in the stress. Experience has however proved that samplessatisfactory for all practical purposes can be obtained provided thatthe natural moisture content and natural structure are not altered.The natural moisture content is maintained by adequate sealingwith amicrocrystalline wax. Structural disturbance can be reducedto a minimum by the use of a correctly designed sampler.

The simplest device for obtaining undisturbed samplesis the thin walled sampler tube. Although brass and hardaluminium tubes are prefened, steel tubes are more common. Toprevent chemical action between the soil and steel tube it should bedipped prior to use in hard, smooth, non-corrosive lacquer. Plasticpaint is sometimes used as an alternative. The tubes should beseamless, clean, with smooth interior free of protrusions. Thecutting end should be machined sharp with an outer taper less than10.: Blunt ended sample tubes should be retumed for machining.The inside clearance between the intemal cutting edge diameterand the intemal wall diameter should not be less than 0.005 (0.57o)

nor greater than 0.03 (37") ofthe internal cutting edge diameter.The lower value is used in partially saturated or soft clays and siltsand the higher value for hard and saturated swelling clays. Theoverall area ratio, (area of outside diameter minus area of insidecutting diameter divided by area of inside cutting diameter ) c" =(D*2 - D"2)/D"2 should not exceed 0.1 (10%).

The sample tube, valves, vents, piston, packing,etc.should always be checked for proper function. The sampler shouldbe lowered carefully into the borehole without dropping. If a

piston device is used the piston should be locked in position priorto pushing the sampler into the ground. The pushing should bedone in one unintemrpted movement without rotation. Arecommended rate is between 150 - 300 fllm per second. Thelength of penetration should never exceed the length of thesampler. Penetrations should also not exceed l0 times the intemaldiameter of the sampler in cohesionless soil and 15 diameters incohesive soils. After penetration the sample should be given timeto adhere to the sampler (usually l0 minutes) and then rotated 2full revolutions before being withdrawn. Samples should behandled and packed for shipment with great care and boxes shouldbe marked fragile and protected form freezing and extreme heat.

Identification should include borehole number, samplenumber, depth, total drive, measured recovery ofundisturbed soilbefore trimming and description ofsoil at upper end oftube. Theratio of the lenglh of soil in the tube before trimming to thedistance pushed.in termed the recovery ratio.

Tests indicate that in most materials,least disturbance iscaused by pushing the sample tube into the ground very rapidly,but it is sometimes impractical to do this. Slowjacking and drivingproduce similar disturbances in most soils except in sensitivematerials where vibration may have appreciable affect.

In soft clays and silty clays, it is better to use the moreelaborate piston samplers. In these samplers, a piston on top ofthesample retains it by suction during withdrawing. The samples areof two basic t1pes, the free piston and the fixed piston, the latterhaving the advantage ofpreventing the entry into the tube ofloosematerial at the bottom ofthe borehole. In addition the standardswedish piston sampler has been designed for use in soft clayswithout a borehole.

Anothertype ofsampleroften used in very sensitive soilsis the Swedish Foil sample. The sample is separated from the sideof the tube by a metal foil which remains stationary while thesampler is pushed into the $ound and thus eliminates frictionbetween the soil sample and the inside of the sample tube.

Undisturbed Sampling in PeatPeat may be treated in a similar manner to cohesive soils

with some changes in technique. Samplers should be as large aspossible, 300 mm diameter or more. Piston samplers are to beprefened. Unlike clays, best sampling results are obtained bygently tapping the sample into the peat moving the sampler 100mm or less for each blow. A sharp cutting edge is essential. Peats

are often very corrosive so all sample tubes should be dipped inhard, smooth, non-corrosive lacquer. A rest time prior to samplewithdrawal is essential.

Undisturbed Sampling Non-cohesive SoilsUndisturbed sampling of sand is a very difficult

operation particularly below the water tab1e. A number ofcomplicated devices have been produced, involving mechanicalmethods ofclosing the end ofthe tube, grouting the sand at the endof the sample, etc. The most successful device is probably theBishop Sampler, which relies on the apparent cohesion producedin sands by surface tension, when they are permitted to drainslightly from a saturated state. This is achieved by withdrawing thesample into a compressed air bell at the bottom ofthe borehole, thetop of the sample tube being sealed by a valve. Disturbance ofsamples ofthis tlpe is almost unavoidable, particularly in handlingat the surface.

When the sample is brought to the surface it is general

to measure the in situ density of the soil and then performminimum and maximum density tests on the non-cohesive sample.Undisturbed sampling of non-cohesive soils is not common.

Hard Clays and Soft and Brittle RocksIt is frequently difficult to obtain good undisturbed

samples of hard clay and weakly cemented rocks with standarddiamond drill core barrels, due to the action ofthe drilling fluidwhich comes in contact with the core before it enters the inner

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sample tube. To overcome this, various tlpes ofbanels have beendevised, having a protruding inner barrel which is generallystationary. A number ofthese are spring loaded, so that the amountofprotrusion ofthe inner barrel is governed by the hardness ofthematerial beine drilled.

Part 2:- FIELD MEASUREMENTS

INDIRECT METHODS OF INVESTIGATIONGeophysical Methods

Where ground conditions are favourable, geophysicalmethods can often be used to obtain quickly over large areas,

information conceming the position and depth of changes in strata.Depending on the method used, they are useful in determination of:thickness of rock and soil strata, location and size of sand andgravel deposits, profiles or rock formations, rippability ofroclgelevation and extent ofwater tables. It is emphasized howeveqthat adequate control borings are essential to correlate and interpretcorrectly the data obtained.

Geophysical methods are successful only when thegeological formation under examination possess markeddifferences in one or other oftheir physical characteristics, and thechoice of method will depend on which of the physical propertiesofthe various strata possess the most pronounced differences. Itis very difficult to interpret the data in complex strata, and inunconformable strata, and geophysical methods of investigationshould be avoided under these circumstances.

Geophysical surveys should always be made by properlyequipped and experienced investigators, as the results are valuelessunless taken with accuracy, and interpretation is of the utmostimportance.

Electrical ResistivityElectrical resistivity is probably the most useful of the

geophysical methods for site investigation. Many strata havedifferent resistances, e.g. clay has a lower resistance than gravel orsand, and some sedimentary rocks have lower resistance thanigneous rocks. In general the denser and drier the material, thehigher its resistance. The presence ofcertain ores may complicatethe results, and resistances vary with water content. Fourelectrodes are driven into the ground, usually at equal distancesalong a straight line- An electric current is passed into and out ofthe ground at the outer electrodes, and the potential differencebefween the two inner electodes is then measured. The depth ofthe block ofground ofwhich the resistance is being measured canbe varied by changing the electrode separation, thus measuring theresistance ofsuccessive strata. The chiefvalue ofthe resistivitymethod lies in the ease, speed, and consequent low cost with whichit can be carried out. Somewhat optimistic claims as to accuracy arefrequently put forward, but correlation with borehole informationis essential ifother than a general picture is required.

Seismic SurveySeismic survey is dependent on measurements ofshock

wave or sound wave velocities. either as a reflection from strata

interfaces, or after refraction through soil and rock masses. Thesound source can be almost anything from a simple hammer blowon a plate, to a large explosion. A number of factors may affect theresults of the worlg for example, gases in the soil pores,particularly prevalent in organic materials,damp out the sandwaves, and penetration can not be obtained. Boulders and largegravel can also cause interference, by reflecting the sound wavesin many directions causing scatter. The sound waves are picked upon geophones ofdifferent types, and the different strata located bytimes taken forthe sound to pass through air direct from the source,and through the soil mass. In general the denser material has a

high sound velocity. The process may also be used over water withsuitably modified equipment, and indeed may be considered to bemore effective in this medium as a continuous profile can beobtained by repeating sound impulses being produced by electricaldischarge in the water, whilst sound source and geophones are

towed along by a boat. Seismic survey methods are generallyexpensive and the interpretation ofthe results is diffrcult, and inconsequence the method is rarely used as a normal siteinvestigation technique. Control borings are essential to obtainany.thing like accurate interpretation.

Dutch Deep Sounding TestIncreasing use is being made in Europe, of the Dutch

deep-sounding device, which is a form of static cone penetrationtest. The apparatus consists principally ofa rod at the lower end ofwhich is a 60..cone of outside area 20 sq. cm. The rod movesfreely inside a tube of the same outside diameter as the cone. Thecone, and then the casing are pushed into the ground at a slow butconstant rate (0.5 to 2 m per minute) by means ofajacking device.The necessary reaction for thejack is produced by kentledge or byscrew anchors, and measured by a proving ring or pressure capsule.

The apparatus is particularly useful as an in situ test insands, in which undisturbed samples are difficult to obtain. Thedepth which can be penetrated by the machine depends on thereaction available and the stiffness or compactness of the soil.Under normal circumstances, very loose sands can be penetrated toa depth ofabout 12 m, but only about 2 m or so in very densesands.

There is a wide experience ofthe empirical relationshipbetween the results obtained, and the bearing capacity andsettlement of foundations on sands. The test has also been used insensitive clay soils, avoiding the disturbance likely in sampling.The test does not bring up a sample, or give data on ground waterconditions, and is reliable only in well defined strata. Controlborings are essential for interpreting the results, and the test is mostadvantageous in providing additional cheap information tosupplement a boring scheme.

In peats modifred static cone is often used with a l0.lmm diameter cone. The force required to push the cone into thepeat in 4.45 N units being related to the bearing capacity andtrafficability of the material.

Dynamic Cone TestThe dynamic cone test is amodification ofthe static cone

test in which a similar cone is driven into the ground using ahammer (usuaily 64 kg in North America) with a free fall of agiven distance (7 62 mm or larger). The number of blows requiredto drive the cone a specified increment ofdepth (generally 305

Site Investigations - GEOTECHNICAL ENGINEERING-1997 .- by G.P" Raymond@ 246

mm) being recorded. The principal advantages of the dynamiccone test over the static test are that no anchorage is needed, andthat deeper penetration is frequently possible. Gravel can bepenetrated by this method, but the results obtained in large gravelare of doubtful value. The test has been used extensively as aguide to pile bearing capacity, and a good deal ofinformation isavailable inthis context. As in the case ofthe static cone test. theuse of this test without adequate borehole control may be verymisleading. It is necessary in both tests that the strata besufficiently defined to enable an accurate assessment to be made ofthe material being penetrated by the test.

Plate Bearing TestThe only in situ test which provides direct information

on the immediate settlement characteristics of a soil is the platebearing test. The test is generally performed in a test pit excavatedto the foundation level. Varying size plates are used, depending onthe ratio which can be obtained, and the loading range it is desiredto investigate. Normal sizes would be in the range 250 mm to 750mm diameter. The load is normally applied by means of ahydraulicjack, and measured by a proving ring or pressure capsule.The reaction is provided by Kentledge, or if the test pit is deepenough to provide the required reaction, by struts into the sides ofthe pit. The load is applied to the bearing plate in increments, andreadings of deflection of the plate relative to a datrm framesupported some distance from the area under test, are taken atspecific time intervals as in a laboratory consolidation test.

The soil immediately below the plate can be examinedbefore test,and the soil to any require depth below the plate afterthe test, giving a complete picture ofthe soil tested. The resultsobtained are directly related to the bearing capacity and settlementoffoundations, and are particularly useful in variable c - q soils,and particularly in fill materials ofvariable content and voids. Infill material, and sand and gravel, the plate bearing test isfrequently the only reliable method of estimating settlements andis frequently used for this purpose.

The principal disadvantage ofthe test is its high cost.Using a 450 mm diameter plate, and 50 tonne of kentledge in a pitI m deep, a test (a series ofload tests) may be expected to takeseveral days to complete. If the expense is to be kept withinreasonable limits, the size of the plate must necessarily be small.In consequence, great care must be taken in applying the results ofthe small scale tests to larger foundations. A boring programme isan essential part of any plate bearing testing, to ensure that soilstrata which will be stressed by the foundations is similar to thattested.

More recently tests have been preformed in stiffclays atdifferent depths in unlined auger holes. This has enabled aconsiderable saving in the costs of healy foundations onoverconsolidated clays.

In peats the test may be performed on a plate augerwhich is augured to any desired depth and then loaded. Again anadjoining borehole is desirable.

Vane Shear TestThe shear strength ofsoft clays can be measured in situ

by pushing into the clay a small four-bladed vane, attached to the

end of a rod, and then measuring the maximum torque necessary tocause rotation. Vanes ofvarious sizes can be used depending onthe strength of the strata being tested, normal sizes being between25 mm and 75 mm diameter, and 50 mm and 150 mm longrespectively. A rotational speed of 6 degrees per minute isnormally recommended. When vane tests were first introduced, adummy rod with no vane on the end was used to measure the shaftfi:iction on the vane rod. Modern apparatus however uses a rodsleeve, which is rotated by itselffirst, the torque being measured,and after about 90 degrees fum, the sleeve and vane tum together.The difference in torque being attributed to the vane. Aftercompletion ofthe undisturbed test, the vane can be rotated to causeremoulding of the soil, and a remould strength measured.

In peat soils the vane has been found to overestimate thestrength in many cases. Small vanes often give higher strengths infibrous peat than large vanes and a minimum diameter of 100 mmis recommended.

IN SITU TESTS IN BORBHOLESVane Shear Test

When used in a borehole, it is normal to push the vanesome 600 rnm beyond the bottom ofthe hole to avoid any disturbedsoil, and in consequence the vane test generally gives higher valuesof shear strength than laboratory tests, in which some disturbancescan be expected.

A number oftheories have been advanced to suggest thatthe vane test can be used in frictional material. Little experienceof this application is available, and it is recommended that its usebe restricted to soft cohesive soils. Due to the comparatively smallsize ofthe vane, interference offered to the vanes by gravel, shellsor small sand pockets may affect the results, and it is consideredessential that adisturbed sample ofthe soil actually tested, be takenfor visual examination.

Standard Penetration TestThe Standard Penetration Test is an empirical test for

sands, made in the bottom of aborehole, and gives a measure of therelative density ofthe sand. The apparatus consists ofa 560 mmlong thick walled 50 mm O.D. tube split longitudinally to facilitatethe removal of the sample, with a standard cutting shoe. Thepenetrometer is driven into the soil by means of a 64 kg hammerwith a free fall of 762 mm. The result of the test, the PenetrationNumber, is the number ofblows required to drive the penetrometera distance of 305 mm. The penetrometer is normally tapped intothe ground for the first I 50 mm to avoid the disturbed area, and thenumber ofblows recorded for 305 mm penetration from this point-As the thickness of the tube is large in comparison with itsdiameter, the sample obtained is rmely undisturbed.

There is extensive experience ofthe relationship betweenthe results ofthis test, and the bearing capacity and settlement ofsands, but the test cannot be used in gravels, and is not generallyapplicable to clays. It is also very sensitive to disturbance inboring, and its inexpert use can result in large errors. The test has

the advantage that used in conjunction with boring, it provides asample for inspection, and allows ground water conditions, etc. tobe recorded.

Cone Penetration TestA development ofthe standard penetration test uses a

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60. rone in place ofthe cutting shoe ofthe standard penetrometer,or a 50 mm diameter steel rod with a 60..cone cut at one end mayequally be employed. The test is carried out in a similar manner tothe Standard Penetration Test, the number ofblows per 300 mmpenetration being the Penetration number. The use of thispenetrometer allows the test to be carried out in gravel strata. Nosample is recovered in this test, for visual examination, and thereis less experience for the relationship between the results and thebearing capacity and settlement of foundations. The test is inprincipal a dynamic cone test, adopted for use in boreholes.

Menard Geocell or Pressuremeter TestAn in situ test that may well have appreciable impact on

future soil mechanics testing is the Menard Geocell orPressuremeter Test. In this test a cell ofslightly less diameter thanthe borehole is lowered to the required depth on hollow rods. Thecell is formed by a cylindrical rubber membrane supported at topand bottom by steel plates. The basic principals ofthe test are thatthe membrane is expanded by fluid pressure to the sides of theborehole, and a series of incremental pressures imposed, duringeach of which, volume change time measurements are taken for thecell. Dummy cells are used above and below the test cells, inwhich the same fluid pressure is exerted. These dummy cellseliminate edge effects on the test cell. By recording volumechange and the radial pressure exerted by the cell on the soil, themakers claim that the bearing pressure and consolidationcharacteristics of the soil can be measured. The orincioaldifficulties with this test are the disturbance of the soil at the sidesofthe borehole during boring, which may be very significant, andthe difficulties of analysing the results due to the complicateddrainage conditions induced in a borehole. Its basic conception ofa means oftesting in situ the principal characteristics of a soil may,with experience, prove of inestimable value to soil mechanics. Thetest is used quite extensively in Europe. It is gradually gainingacceptance in North America.

Permeability TestsUnder flood control embankments and in dam

foundations the permeability ofthe foundation material is ofutmostimportance. Also of importance is the permeability of soils fordewatering systems and consolidation tecbniques. Because thesecondary structure ofin situ soils, stratification and cracks havea great influence on the permeability, only the order ofmagnitudeof the field permeability is obtainable. Several types of in situtechniques are used depending on the soil conditions.

Pumping tests require the installation of a centralpumping well plus piezometers at different radii to observedrawdown water levels. These tests are used to predictrequirements for large scale dewatering of construction sites orpermanent pressure reliefoflarge structures. They have also beenused in peat deposits for predicting drainage requirements for peatharvesting.

Variable head permeability tests are generally performedin an open borehole or open tube piezometer depending on thepermeability of the soil. The standpipe water is raised or loweredliom equilibriurn and allowed to recover while readings are madeof excess head (above or below static) versus time. Both soilanisotropy and the shape ofdrainage surface (piezometertip) affectresults. If anisotropic effects are to be investigated severaldifferent drainage length to diameter ratios should be used.

Constant head permeability tests are used to obtain thepermeability of fine grained soils. The excess head is normallyraised and maintained constant so as to flow tkough a volumemeasuring device to the top of a piezometer. A two leadpiezometer is recommended so that air or gas can be flushed fromthe leads and the excess head measured on the retum lead toeliminate head loss due to velocity flow. Care must be taken thatthe excess head does not cause hydraulic fracture and should thusbe less than halfthe effective overburden pressure. In additionafter measuring the in situ permeability the head may be increaseduntil a large increase in permeability is recorded. The headrequired to initiate the large increase is associated with hydraulicliacture of the ground. This head has been suggested as beingequal to the in situ lateral eflective stress, giving a measure ofthecoefficient ofearth pressure. Ifthe ground surface is horizontalthis value can be related to the'at rest'value for the degree ofoverconsolidation at the test deoth.

Other TestsThere are a large number ofother in situ tests which are

commonly used, in practice, generally for the investigation ofspecific problems. Of these, probably the most widely used are insitu density and moisture content estimations for compactioncontrol, and also California Bearing Ratio tests for pavementdesign. Whilst these are not normally used in foundationinvestigations, they may on occasion provide useful information.It is suggested that wherever possible as much information as cuur

be economically obtained be derived from in situ testing. In situtests are frequently cheaper to perform than the equivalentlaboratory tes! and can, in many cases, give results underconditions more nearly approximating those under which the soilis to be treated as an engineering material.

REFERENCES

Hvorslev, M.J., 1949. Sub-surface exploration and sampling ofsoil for civil engineering purposes. Soil Mech. Found. Div. Am.Soc. Civil Engrs.

Soil Test, Sub-surface Investigation Catalogue.

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TABLE 1. REQUIREMENTS FOR BORING LAYOUT, FROM NAVAC DM-7

Areas for Investisation Boring Lavout

New site of wide extent. Space preliminary borings so that area between any four borings includes approximatelyl0% oftotal area. In detailed exploration, add borings to establish geological sections atthe most useful orientations.

Development of site on softcompressible strata.

Space borings 30 to 60 m at possible building locations. Add intermediate borings whenbuildine sites are determined.

Large structure with separate

closely spaced footings.Space borings approximately 15 m in both directions, including borings at possibleexterior foundation walls, at machinery or elevator pits, and to establish geologic sectionsat the most useful orientations.

LowJoad warehouse buildinsoflarse area.

Minimum of four borings at corner plus intermediate borings at interior foundationssufficient to define subsoil orofile.

Isolated rigid foundation,225 to900 m2 in area.

Minimum of three borings around perimeter. Add interior borings depending on initialresults.

Isolated rigid foundation, lessthan225 m2 in area.

Minimum of two dry sample borings at opposite corners. Add more for erratic conditions.

Major waterfront structures,such as drv docks.

Ifdefinite site is established, space borings generally not farther than 30 m addingintermediate borings at critical locations, such as deep pump well, gate set, tunnel orculverts.

Long Bulkhead or wharf wall. Preliminary borings on line of wall at 120 m spacing. Add intermediate borings todecreasing spacing to 30 or I 5 m. Place certain intermediate borings inboard andoutboard of wall line to determine materials in scours zone at toe and in active wedeebehind wall.

Slope stability, deep cus, highembankments.

Provide three to five borings on line in the critical direction to establish geological sectionfor analysis. Number of geological sections depends on extent of stability problem. Foran active slide. place at least one boring upslope of slidins area.

Dams and water retentionstructures.

Space preliminary borings approximately 60 m over foundation area. Decrease spacing oncentre line to 30 m by intermediate borings. Include borings at location of cutoff criticalsoots in abutrnent.

Highways and airfrelds. See NAVFAC DM-5 and NAVFAC DM-21 for highways and air fields. For slopestability, deep cuts, and high embankments, see layout recommended above.

Site Investigations - GEOTECHNICAL ENGINEENNG-l997 -- by G.P. Raymond@ 249

TABLE 2. REOUIRBMENTS FOR BORING DEPTH. FROM NAVAC DM-7

Areas for Investisation Borins Depth

Large structwe with separateclosely spaced footings.

Extend to depth where increase in vertical stress for combined foundations is less that l0%of effective overburden stress. Generally all borinss should extend no less than

Isolated rigid foundations. Extend to depth where vertical stress decreases to l0% ofbearing pressure. Generally allborings should extend no less than 9 m below lowest part offoundation unless rock isencountered at shallower deoth.

Long bulkhead or wharfwall. Extend to depth below dredge line between 314 and l-ll2 times unbalanced height of wall.Where stratification indicates possible deep stability problem, selected borings shouldreach too of head stratum.

Slope stability. Extend to an elevation below active or potential failure surface and into hard stratum, or toa depth for which failure is unlikely because ofgeomefy ofcross section.

Deep cuts. Extend to depth between 314 and I times base width of narrow cuts. Where cut is aboveground water in stable materials, depth of 1 to 3 m below base may suffice. Where base isbelow elround water" determine extent oforevious strata below base.

High embanknents. Extend to depth between ll2 and l-ll4 times horizontal length of side slope in relativelyhomogeneous foundation. Where deep or irregular soft strata are encountered, boringsshould reach hard materials.

Dams and water retentionstructures.

Extend to depth of l/2 base width of earth dams or 1to 1-ll2 times height of smallconcrete dams in relatively homogeneous foundations. Borings may terminate afterpenetration of 3 to 6 m in hard and impervious stratum if continuity of this stratum isknown from reconnaissance.

Highways and airfields. Extend auger borings to 2 m below top ofpavement in cuts, 2 m below existing ground inshallow fills. For hieh embankments or deeo cuts. follow criteria siven above.

Site Investigations - GEOTECHNICAL ENGINEERING-1997 - by G.P. RaymondO 250